In the periodic scanning of a certain space in range and azimuth, by an antenna of a monopulse radar feeding incoming echoes to a two-dimensional display screen, problems are often encountered on account of spurious pulses due to so-called "angel echoes" caused by a variety of reflecting objects whose presence is of no interest to the observer. Thus, for example, the monitoring of aircraft in an aerial-navigation system will be impeded by such spurious reflectors as surface vehicles, flights of birds, updrafts of warm air, evaporation from large bodies of water and other atmospheric phenomena. Though the well-known moving-target indicators (MTI) blank out stationary objects, this technique is not effective against false targets moving at low or moderate speeds in a radial direction as seen from a control tower or other observation post equipped with such a radar.
These spurious reflectors are generally characterized by rather limited absolute velocities (on the order of tens of kilometers per hour) and a fairly dense but nonuniform spatial distribution. The resulting "angel echoes" may exhibit low-frequency fluctuations (on the order of tens of Hz) and slow overall variations with time; their signal-to-noise ratio (SNR) may be comparable to that of small aircraft having reflecting surfaces of several square meters.
The usual solutions for dealing with the problem of "angels" are designed to modify the amplification of incoming echo pulses, as by the known STC (sensitivity time control) system, or the elevational angle of the antenna beam. Since, however, the locations of the spurious reflectors within the surveyed space are not in advance, these modifications cannot be localized in range or in azimuth and therefore also affect the true echo signals. Radar receivers of the so-called CFAR (constant false-alarm rate) type implement a not entirely satisfactory compromise between the needs for a comprehensive surveillance and the desire to minimize interference from spurious echoes. Manually setting the rates of amplification or attenuation for pulses received from different range bins, on the basis of visul observation of cluttered areas on the screen, is a somewhat cumbersome procedure liable to distract the attention of the operator.